Electrolytic method of winning zinc



Patented July 8, 1952 ELECTROLYTIC METHOD OF WINNING ZINC Geofirey J. J. Mould, Flin Flon, Manitoba, Canada, assignor to Hudson Bay Mining and Smelting' 00., Limited, Winnipeg, Manitoba, Canada.

a corporation of Canada Application September 7, 1948, Serial No. 48,150

.. No Drawing.

v 3 Claims. 1' This invention relates to addition agents for electrolytic cells and to the method of obtaining higher current efi'iciencies by the use of them when metals are plated out of solution. More particularly it relates to a method of obtaining a higher current efliciency when zinc is plated out from zinc electrolytes used in the electrolytic method of winning zinc, from ores or othermaterials. 7

It is well known in the art of plating metals from electrolytes that the amount of metal deposited by a certain amount of electricity is below theamount to be expected on the basis of Faradays law. Thus, in, the plating of zinc from a zinc sulphate solution as is usual in the electrolytic method of winning zinc the amount of zinc deposited from what may be termed a good commercial electrolyte ranges from 85% to 95% of that calculated according to Faradays law, the deficiency resulting from the evolution h r e m 7 The current efiiciency obtained in the plating of Zinc is of great commercial importance, because .ofthe cost o f electric current, more current beingrequired per pound of zinc when the efficiency is low and particularly because when thefefficiency islow the zinc is not deposited in a, compact sheetbut isfull of holes. Such zinc is difficult to handle in the stripping and melting operationswhich follow. Moreover, when the Zinc is deposited at low current efficiencies the zinc contains more impurities than is the case where the current efiiciencies are high. In the, normally known methods of winning zinc electrolytically the, zinc obtained is comparatively poor, being; rough and porous, and no means has been discovered, so far as is known up to the date of, the present invention, to overcome this diificulty. g 7

Various factors have ya deleterious effect on current efiiciency'and the best known and most important is the presence of certain'impurities in the electrolyte such as antimony, arsenic, cobait, iron,' nickel, germanium, tin, selenium, tellurium, copper, the platinum metals and, according to some investigators, silicon.

The difficulties above enumerated have been overcome by the present invention according to which an,- improvement is obtained in the current efficiency, of zinc deposition by adding small amounts of rosin soap .(a sodium rosin soap), to zinc ,electrodeposition cells in amounts, of three-quartersof a pound .up to ten pounds per ton ofvcathode zincto be deposited but normally in amounts of .theordercof one and a half pounds per ton of cathode zinc to be deposited. The addition of a reagent to an acid zinc electrodeposition cell for the purpose-of causing the zinc to become non-wettablegprotects the zinc from the acid anda better product is obtained. If a rosin soap is added to the electrolyte, the zinc is rendered resistant to corrosion and the addition of the soapwithinjlimits leads to increased current efiiciency.

In order to illustrate the invention reference is made to the following examples of experiments which have been'performed.

Samples of certain oils, wax'esand resins, set forth below, were saponified by heating with a solution of sodium hydroxide until reaction had ceased, and then the soap compounds from the resultant mixture were separated by salting out, a method well known in the art of soap-making. To ensure that the material obtained was more or less completely saponified, this same material was again heated together with more sodium hydroxide solution, and the soap compounds again were separated from the resultant mixture by salting out.

The product obtained was dried and converted into a water solution such that ml. of solution contained one gram of the dried soap.

The various soaps were tested in zinc electrodeposition cells of a standard type, the cathodes being aluminum, and. the anodes lead, spaced so that the surfaces of the electrodes were about one inch part. 'Iv'he-current was adjusted so that the cathode current density was about thirtysix amperes per square foot. The feed introduced to the cells at the bottom contained about grams per litre of zinc as sulphate solution, and the rate of feed was matched with the rate of zinc deposition, the operation being so carried out that the stripped solution which overflowed at the'surface of the electrolyte contained approximately 120 grams per litre of sulphuric acid. i

Every twenty-four hours the cathode was removed from the cell, and the zinc plated thereon removed and dried and Weighed.

When soap solutions prepared as already described were added daily 'to, 'such a cellin the amount'of 5 ml. per litre of electrolyte per day, the efficiency of zinc electrodeposition was very noticeably improved. 7

The details of one set of such tests, in which four cells, such as have beenfdescribed, were operated together in such a manner that the current passing through all the cells was exactly the same, appear below. .i

To one cell, referred to as cell No. 1 was added after stripping each day, 5 ml. of soap solution per litre of electrolyte, the soap solution having been prepared from rosin in the manner already described.

Similarly, to cell No. 2 was added after stripping each day, 5 ml. per litre of soap solution prepared from croton oil.

Similarly, again, to cell No. 3 was added after stripping each day, 5 ml. per litre of soap solution prepared from castor oil.

Cell No. 4 was a cell to which no additions were made and it was used in the experiment for comparison purposes.

Each of these cells was operated in exactly the same manner, and the solution fed to each cell was exactly the same, containing, besides zinc sulphate, impurities as found in the electrolyte as fed to the electrodeposition cells in the electrowinning of zinc. A portion of this solution was assayed and the following amounts of certain impurities were found:

Cobalt, 10.3 milligrams per litre. Arsenic, .05 milligrams per litre. Antimony, .04 milligrams per litre.

Under these conditions, and over a period of ten days the following results were obtained:

Cell No. 4: Cell for comparison purposes. N o addition.

Cell No. 1: Cell No. 2: Cell No. 3: Daily addi- Dally addi- Daily additions 01' 6 tions of 5 tions of 5 ml. er litre ml. per litre ml. per litre ros soap crotonsoap castor soap soln. soln. soln.

Per Cent Av. C. E Av. Pb Assay. Av. Cu Assay Av. Cd Assay The results of other experiments are given hereunder.

Per Cent of Pilot Yield or y Zinc Yield (8 Test N0. Kind of Soap Remarks Toilet or hand soap (common, hand milled).

Linseed Soap..'...

Rosin Soap Pilot mg/l 9.9 .02 .03

Fairly good electrolyte 00 As Sb mg/l 10.1 .03 .026 Switched over to less pure electrolyte about half way through this test. 129-2 Co As Sb mg/l Beeswax Soap.--. Soyabcan Soap.. Rapeseed Soap Pilot Same feed as used in latter part of previous test.

Rosin Soap Croton Soap Castor Soap 126. Pilot The eflect of change of electrolyte can be seen in the last group where an impure electrolyte was used throughout the test and in the next to last test in which rather impure electrolyte was used in the latter part of the test.

Soaps from beeswax which contain palmitic derivatives and melissyl alcohol and soaps from resin were found to give the smoothest plates. The judicious addition of wetting agents has an effect in smoothing the plates. I

Fairly good electrolyte Co As Sb Suitable ranges and the preferred amounts of various soaps tried are as hereunder:

Gardinol is an alkyl sodium sulphate containing the lauryl radical and/or the myristyl radical. Both lauric acid 01211240: and myristic acid 0141-12802 are normal saturated monobasic acids. The structure of Gardinol may be represented by the formula where R is the organic radical.

Another agent which may be used is Duponol which is a higher alkyl sulphate made from a mixture of lauryl and oleyl alcohols. Oleic acid is an unsaturated monobasic acid of composition C1sH34O2.

The smoother zinc sheet obtained with the use of beeswax soap is thought to be due to the alcohol modifying the waterproofing action of the palmitic soap. Therefore the alcohol acts more or less as a. wetting agent.

It is not easy to get zinc to plate onto the oathode and to keep it there without its redissolving. A water shedding film of soapy material more or less prevents the acid in the cell from attacking the zinc on the cathode. On the other hand this same film of soapy material may make the process of plating onto the cathode a harder thing to do due to the material acting as an electrical insulation. There are therefore two factors which have to be balanced, one against the other to obtain the maximum yield of zinc. It does not follow therefore that the soap which has the most waterproofing efiect is necessarily the best for increasing the current emciency. The best known soaps at the moment for obtaining the desired results are common rosin soap and beeswax soap. Beeswax soap'is relatively expensive as compared with common rosin soap but both give good plates and the purity of the zinc deposited is improved. Used in amounts of one to one and a half pounds per ton of cathode zinc deposited they are very satisfactory.

The embodiments of the invention in which an exclusive property of privilege is. claimed are defined as follows: A

1. In a process of winning zinc which comprises electrodepositing zinc from a zinc sulphate electrolyte, the improvement which consists in adding a sodium rosin soap to the zinc sulphate electrolyte in an amount eflective to increase the current efficiency. I

2. The improvement in a process of winning zinc as defined in claim 1 wherein the amount of sodium rosin soap added to said electrolyte varies between three-quarters of a pound and ten pounds per ton of cathodezinc to be deposited.

3. The improvement in' a process of winning z'mc' as defined in'claini lwherein the amount of sodium rosin soap added to said electrolyte is about one and one half pounds per ton of cathode zinc to be deposited.

GEOFFREY J. J. MOULD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,876,156 Thompson et a1. Sept. 6, 1932 1,895,675

Mathers Jan. 31, 1933 6 Number Name Date 2,113,517 Powell Apr. 5, 1938 2,389,135 Brown Nov. 20, 1945 2,389,179 Brown Nov. 20, 1945 2,389,180 Brown Nov. 20, 1945 OTHER REFERENCES Transactions of the American Electrochemical Society, vol. XLV (1924), pages 414, 415, 420, 428. Silman: Journal of Electrodepositors' Technical Society, vol. 19 (1943-44), pages 131-134, 

1. IN A PROCESS OF WINNING ZINC WHICH COMPRISES ELECTRODEPOSITING ZINC FROM A ZINC SULPHATE ELECTROLYTE, THE IMPROVEMENT WHICH CONSISTS IN ADDING A SODIUM ROSIN SOAP TO THE ZINC SULPHATE ELECTROLYTE IN AN AMOUNT EFFECTIVE TO INCREASE THE CURRENT EFFICIENCY. 